The invention is directed to a device, for detecting polarization mode dispersion of an optical data signal, which includes a filter to whose output a power detector is connected.
Long light waveguide transmission links are utilized in optical transmission technology. The light waveguides, as a result of manufacture, are not completely isotropic but slightly birefringent. Due to the long transmission link, a frequency dependent polarization transformation occurs, which is called polarization mode dispersion (PMD) or simply polarization dispersion. This, due to modification of the polarization of the optical signal as a function of the optical frequency and associated different, frequency-dependent running times, leads to the spread of the transmitted pulses, resulting in a reduced recognizability of these pulses at the reception end and, thus, the transmitted data rate is limited.
An additional complicating factor is that the transmission behavior of the link and, thus, the PMD as well, changes due to temperature modification or mechanical stressing. Adaptive PMD compensators are therefore utilized that are inserted into the transmission path. PMD distortions must be detected in the optical receiver for the drive of these compensators. The compensator, for example, can then be optimally set with a gradient algorithm.
Electronic Letters, 17 Feb. 1994, Vol. 30, No. 4, pages 348 through 349, discloses the use of a band-pass filter for filtering a data signal whose PMD is to be detected. A power detector at the filter output supplies a signal that is higher the lower the PMD distortions are.
It is disadvantageous that, when high PMD of the first order is present, this signal does not vary monotonously as a function of the differential group delay DGD and unambiguous signals therefore can not be acquired.
Proceedings OEC 94, 14e-12, pages 258 through 259, Makuhari Fare, Japan 1994, discloses the employment of a different method in which the power of the difference signal between the decision unit output and the decision input is interpreted. This signal, however, has a lower sensitivity to PMD distortions than a suitable band filter. Moreover, false decisions can occur, particularly for great PMD distortions where the DGD exceeds the bit duration, so that the acquired signal is an unsuitable criterion in such instances for the presence of PMD distortions.